The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube provided with a shadow mask corresponding to a large-screen flat face.
Color cathode ray tubes which are widely adopted as display devices for information terminals and television sets have a tendency for their screen sizes to become large. In addition, a so-called flat-face type in which a panel portion which forms a screen is flattened has been becoming popular.
The current popular type of color cathode ray tube is constructed in such a manner that a color selection electrode is installed close to the inner surface of a panel on which phosphor picture elements for three colors are formed, and three electron beams emitted from an electron gun are made to individually impinge on the respective phosphor picture elements by means of the color selection electrode.
Such a color selection electrode uses a so-called pressed mask having a structure in which a thin plate member in which multiple holes (electron beam apertures) such as round holes (dot-like holes) or elongated holes (slots) are formed is press-formed into a curved shape which follows the curvature of an inner surface of a panel portion, or a tension mask having a structure in which a thin plate member in which multiple holes are formed is tensely mounted on a frame-like member (frame), or a so-called aperture grill having a structure in which multiple strings arranged in the form of a reed screen are tensely passed between opposite sides of a frame-like member. Incidentally, the pressed mask and the tension mask in particular are also called shadow mask.
This kind of color cathode ray tube has an evacuated envelope which is integrally formed of a panel constituting a phosphor screen, a neck accommodating an electron gun, and a funnel-like funnel connecting the panel and the neck.
Incidentally, an art associated with the structure of an apertured region where the electron beam apertures of the shadow mask are disposed is described in, for example, Japanese Patent Laid-Open No. 82234/1996.
The panel portion of the flat-face type of color cathode ray tube has nearly flat inner and outer surfaces and has a glass plate thickness which is made large compared to the panel portion of a round-face type of color cathode ray tube in order to ensure mechanical strength such as implosion resistance. In general, the above-described tension mask is used as a shadow mask to be installed in this flat-face type of cathode ray tube.
The panel portion of another flat-face type of color cathode ray tube has a nearly flat outer surface and has an inner surface which is curved in a concave shape toward the outer surface so that its glass plate thickness is made larger in its peripheral portion than in its central portion. A pressed mask, which is press-formed in a shape approximately following the curvature of the inner surface of the panel portion, is widely used as a shadow mask to be installed in this flat-face type of cathode ray tube.
As a panel which constitutes the evacuated envelope of a color cathode ray tube, a panel (tint panel) which uses a tint material suited to high-contrast picture display is widely used. At present, this tint panel is the main current in the market of panels for cathode ray tubes, and is comparatively inexpensive in terms of cost and can be easily obtained. In addition, the tint panel is superior in the contrast of pictures in cathode ray tubes because the tint panel is low in optical transmissivity compared to a panel using a clear material or a gray material (a clear panel or a gray panel).
On the other hand, in the case of a panel in which the radius of curvature is made extremely larger on the outer surface than on the inner surface and the outer surface is made nearly flat, the thickness of the panel is considerably larger in its peripheral portion than in its central portion, and the optical transmissivity (optical transmission amount) of the peripheral portion is considerably lower than that of the central portion. If a tint panel which is low in optical transmissivity as a whole is applied to the above-described panel which is made nearly flat, a lowering of the optical transmissivity of the peripheral portion stands out, and the uniformity of luminance over the entire region of the screen of the cathode ray tube is impaired.
Accordingly, if the emission amount of phosphors is the same over the entire region of the panel, i.e., if an electron beam amount to be transmitted through a color selection electrode is the same over the entire region, the brightness of the peripheral portion lowers. This difference in optical transmissivity can be corrected by changing the electron beam amount to be transmitted through the color selection electrode.
In addition, in the flat-face type of color cathode ray tube in which the inner and outer surfaces are made nearly flat, particularly in the flat-face type whose screen size is large, a tension mask or an aperture grill is suitable in which deformation due to thermal expansion at the time of collision of electron beams with the color selection electrode does not easily occur.
However, the tension mask and the aperture grill, when color selection electrodes are to be tensely mounted on their frames, require a process for imparting an optimum tension taking account of a relaxation amount or the like due to material strength and temperature variation, and need structures which are mechanically rigid as the frames. Therefore, the manufacturing costs of the tension mask and the aperture grill are large.
In contrast, since the pressed mask is manufactured by being press-formed into a shape approximately following the curvature of the inner surface of the panel, the manufacture of the pressed mask per se is easy. However, on the other hand, as the panel size is larger, for example, as the diagonal length of the effective screen region of the panel becomes larger, to 76 cm or more, the curvature of the central portion of the inner panel surface becomes smaller (the radius of curvature becomes larger). In the case where this pressed mask is applied to a panel whose outer surface is made nearly flat, the difference in plate thickness between the central portion and the peripheral portion needs to be made as small as possible in order to improve the uniformity of luminance over the entire region of the screen of the cathode ray tube. Specifically, the extent of curvature of each of the inner surface and the pressed mask needs to be made as small as possible, and the radius of curvature of each of the inner surface and the pressed mask needs to be made a predetermined value or more. In general, in the case of the pressed mask which is formed of a thin plate, as the extent of curvature of the pressed mask becomes larger, its shape holding ability becomes larger, whereas as the extent of curvature becomes smaller, its mechanical strength becomes lower and the shape holding ability becomes smaller.
In particular, if the pressed mask is used in the above-described flat-face type of color cathode ray tube having a very large screen size, the pressed-mask strength of the central portion in which the radius of curvature is large becomes small and the mechanical strength (curved-surface-shape holding ability) easily becomes low, so that shape deformation occurs due to external shock during a manufacturing process or during transportation or due to heating during operation and causes a great lowering of color reproducibility, and a lowering of picture quality is liable to occur.
The curvature of this kind of pressed mask is made small in the center portion and large in the peripheral portion, and the pressed mask is fixed to the frame-like member by being bent along its outermost skirt portion in a direction parallel to the tube axis of the color cathode ray tube.
The zone of the above-described central portion exists in a width of approximately ⅓ to ⅔ in the center of the apertured region in the long-axis direction thereof which region is a region containing a portion where the radius of curvature of the pressed mask is the largest. For this reason, the peripheral portion can acquire the mechanical strength required to hold its shape, but in the central portion, it is difficult to obtain sufficient mechanical strength.
From these facts, if the pressed mask is used in the flat-face type of color cathode ray tube having the tint panel, there occurs the problem that it is necessary to ensure the mechanical strength of the central portion.
The invention solves such problem and realizes a high-quality flat-face type of color cathode ray tube by combining a tint panel with a pressed mask. In particular, the invention solves the problems peculiar to the pressed mask in a large-sized or very-large-sized flat-face type of color cathode ray tube using a tint panel, thereby obtaining a shadow mask whose deformation resistant strength is large.
To this end, in a typical constitution of the invention, the outer surface of the panel of the color cathode ray tube is made approximately flat and the inner surface of the panel is curved to become concave toward the outer surface, and at least one of the long-length direction pitch of slots formed in a pressed mask to be disposed close to the inner surface and the connection-direction width of each of bridges which connect the slots in the long-length direction is made different between the central portion and the peripheral portion of the apertured area of the pressed mask, whereby the ratio of bridges per unit area is made larger in the central portion than in the peripheral portion or the opening rate of slots per unit area is made larger in the peripheral portion than in the central portion.
In general, if the radius of curvature of the central portion of the pressed mask is made large, the space between the inner panel surface and the central portion varies due to a so-called doming phenomenon due to the application of external shock or temperature rise, and undesired deformation of the pressed mask occurs.
However, according to the typical constitution of the invention, the mechanical strength of the central portion of the pressed mask is made large, whereby it is possible to restrain the occurrence of the undesired deformation of the pressed mask. In addition, it is possible to uniformize brightness over the entire screen.
The central portion can be defined as a zone having an approximately ⅓ to ⅔ width in the center of the panel in the long-axis direction thereof, or a zone having an approximately ⅓ to ⅔ width in the center of the panel in the short-axis direction thereof, or a combination of these zones. Moreover, slots can be provided between the central portion and the peripheral portion at a long-length-direction pitch which is an intermediate value between those of the slots in the central and peripheral portions, and an intermediate portion (transitional portion) having the connection-direction space width of each of bridges which connect these bridges can be provided. In addition, in at least one of the central portion and the peripheral portion, the slot pitch and the bridge width can also be gradually varied in a direction away from the center of the apertured region. With this constitution, sharp variations in the slot pitch and the bridge width are restrained, whereby mechanical strength can be smoothly varied from the peripheral portion to the central portion.
Accordingly, even in the case of a very-large-sized flat-face type of color cathode ray tube in which the diagonal size of the effective display region exceeds 76 cm, a tint panel and a pressed mask can be combined and a low-cost and high-quality color cathode ray tube can be realized.
Incidentally, the invention is not limited to a flat-face type of color cathode ray tube having a large screen size or a very large screen size, and can also be similarly applied to a flat-face type of color cathode ray tube having a comparatively small screen size, in the case where a very thin plate is used as its pressed mask.